1. Technical Field
This invention concerns the field of communication satellites and, more specifically, the problem of providing a practicable satellite that is capable of acting as a satisfactory replacement satellite for the majority of Fixed Satellite Service ("FSS") communications satellites that are in orbit and desirably also for the majority of such satellites that are to be placed in orbit.
2. Background
Communications (or telecommunications) satellites have been used for many years. Uplink signals are sent by one or more Earth stations, received by one or more uplink antennas on the satellite, processed by circuitry in the satellite (e.g., frequency-shifted and amplified), sent back (retransmitted) to Earth by one or more downlink antennas on the satellite, and received by one or more Earth stations. The satellites may be placed in various orbits around the Earth. One particularly desirable orbit for certain communications satellites is an equatorial orbit (that is, substantially in the plane of Earth's equator) at an altitude of approximately 22,300 miles. In that orbit at that altitude, the period of revolution of the satellite around the Earth is equal to the period of rotation of the earth. Accordingly, transmitting (uplink) and receiving (downlink) stations on Earth "see" the satellite remaining at a fixed point in the sky and, thus, the satellite may be considered to be in a geosynchronous equatorial orbit or to be geostationary. As a result, a geostationary satellite's position can be defined by its equatorial longitude. For example, satellites useful for broadcast to the continental United States and its territories may be located from about 69 degrees west longitude to about 139 degrees west longitude.
One advantage of using a geostationary satellite is that the transmitting and receiving stations on Earth do not need to track a satellite in a preselected orbital slot across the sky to maintain the desired uplink and downlink communications characteristics (strength of the signals received by the satellite, footprint of the downlink signals on Earth, etc.). In other words, the antennas on a geostationary satellite can be fixed (or stationary) and the footprints of the downlink antennas can also be fixed.
In addition to typically having fixed antennas, geostationary satellites also typically are designed to receive certain signals on preselected frequency bands (the uplink bands) from one or more preselected geographic areas on Earth according to the uplink frequency plan, to amplify the signals to the desired power level, and to retransmit them down to Earth on other preselected frequency bands (the downlink bands) to one or more preselected geographic areas on Earth according to the downlink frequency plan.
Unfortunately, as is well-known, there is a significant probability of a malfunction or complete failure during the launch sequence, and even after a successful launch, there may be a problem while trying to deploy the satellite in the desired orbital position (slot). Failures may also occur after the satellite has been successfully positioned in its slot and operated for a period of time. Failures include sudden or gradual, partial or complete loss of telecommunications capability.
In view of the serious economic loss that can result from not having a fully and properly functioning telecommunications satellite operating in its slot throughout the entire expected time period, it is desirable to provide a replacement satellite (i.e., a spare or back-up satellite) that can assume the telecommunications functions of a failed satellite. Replacement satellites may be stored in orbit or on the ground, and each mode of storage has advantages and disadvantages. Regardless of which storage mode is used, because of cost, weight, and other considerations, the replacement satellite will typically be designed for the same uplink and downlink frequency plans, power levels, footprints, telemetry and command subsystem frequencies, etc. as of the satellite for which it is designed to be the spare.
The substantial cost of spare satellites represents a significant expense for providers of satellite communications channels (e.g., organizations owning satellites and leasing their channels for retransmission). That is particularly true because the spare may not ever be needed. Therefore, it would be highly advantageous if such providers could avoid or at least substantially reduce that expense.
Various methods of providing spares have been proposed. See, e.g., U. S. Pat. Nos. 3,995,801, 5,120,007, and 5,813,634. Other documents concerning or mentioning spare satellites, back-up coverage, and/or replacing a failing or failed satellite include U.S. Pat. Nos. 4,502,051, 5,289,193, 5,410,731, and PCT WO 98/04017. Other documents concerning communication satellites, communication systems comprising constellations of satellites, communication satellite subsystems and components thereof, and methods of operating communication satellites and systems include U.S. Pat. Nos. 4,688,259; 4,858,225; 4,965,587; 5,020,746; 5,175,556; 5,297,134; 5,323,322; 5,355,138; 5,523,997; 5,563,880; 5,860,056; and 5,890,679; EPO Published Application EP 0 915 529 A1; F. Rispoli, "Reconfigurable Satellite Antennas: A Review," Electronic Engineering, volume 61, number 748, pages S22-S27 (April 1989); and Electronics Engineers' Handbook, Section 22-63, "Satellite Communications Systems," pages 22-61 to 22-62 (1975). (All of the foregoing documents and any other documents discussed or otherwise referenced herein are incorporated herein in their entireties for all purposes.)
Some of those documents concern movable antennas. See, e.g., EP 0 915 529 A1.
Some of those documents concern reconfigurable satellites. See, e.g., U.S. Pat. Nos. 4,688,259; 4,858,225; 4,965,587; 5,175,556; 5,289,193; 5,355,138; PCT WO 98/04017; EP 0 915 529 A1; and F. Rispoli: "Reconfigurable Satellite Antennas: A Review," Electronic Engineering, volume 61, number 748, pages S22-S27 (April 1989). Some of those documents concern moving satellites, e.g., from one slot to another or for station-keeping. See, e.g., U.S. Pat. Nos. 5,020,746; 5,813,634; and PCT WO 98/04017.
Replacement satellites that are essentially perfect spares (or clones) for essentially all FSS (C band/Ku band) communications satellites may have been considered by others, but, as far as is known, were never built, probably because they were impractical and/or were prohibitively expensive. The problem of providing such a satellite is made all the more complicated by the fact that the conventional C band/Ku band communications satellites have widely differing characteristics concerning, for example, the uplink and downlink communications frequencies used, power levels, and coverage patterns. Furthermore, conventional satellites well before being launched and put in orbit have been designed for particular orbital slots having neighboring satellites with known telemetry and command frequencies and other characteristics.
Accordingly, there has been a long-standing need for practicable but satisfactory replacement satellites for C band/Ku band communications satellites (FSS satellites). In other words, there has been a long-standing need for practicable C band/Ku band replacement satellites that can emulate the performance of a substantial percentage (and preferably a very high percentage) of orbiting C band/Ku band communications satellites while still being technologically, economically, and otherwise practicable.